Photoinitiated reactions

ABSTRACT

Disclosed is a method for the photoinitiated transformation of a transformable reactive substrate. The method includes an initial step in which a protected ketone photoinitator species which is present in the suhstrate is deprotected to form the corresponding ketone photoinitiator species for use in a subsequent photoinitiated reaction in the method. The ketone group of the photoinitiator is protected by an unsubstituted 1,3 dioxolanc group. Also disclosed are a composition which may be used in the method, the use of the protected ketone photoinitiator in a photoinitiated reaction, as well as the protected ketone photoinitiators themselves.

This invention relates to photoinitiated reactions, for example curingand polymerisation reactions carried out under irradiation byelectromagnetic radiation. More particularly it relates to UV curing asused in photopolymer technology, as well as photoinitiated colourforming reactions.

BACKGROUND

Within the growing realm of UV curing technology, one of the mostimportant applications is in photo-imaging. Ever since the birth ofphotography, new and innovative ways of obtaining images by exposure tolight have been explored; even the silver halide process itself, whichstill forms the core of non-digital photography, has undergonesubstantial change, as for example when the introduction of T-grainemulsions took place.

Although substantial improvements in photopolymer technology have beenmade over the last 20 years, the sensitivity of the processes is stillvery limited compared with the photosensitivity of the silver halideprocess. One of the major goals in photopolymer science is to approachthe sensitivity of silver halide based processes.

Two basic methods of increasing the photopolymer quantum yield beyondunity exist. The first of these is most familiar as the acrylatechemistry used in most commercial free radical UV cure systems. Theapproach here is that of the chain reaction in which one or morephotoinitiators that are exposed to electromagnetic radiation of asuitable wavelength/energy, absorb photons incident upon them. Theenergy of the photon is used chemically by the photoinitiators togenerate free radicals in the irradiated substrate, each of which isthen capable of causing many polymerisable molecules to polymerise veryquickly resulting in a high quantum yield of polymerisation. Thus thequantum yield for this process is high but still not as high as thatoverall for silver halide photography.

The second fundamental form of photopolymer quantum yield enhancement isexemplified by the cationic UV curing systems. In this instance, theabsorbed photon generates a catalytic monomer species which is capableof catalysing polymerisation, cross-linking, or even molecular cleavage.This technology has been described as capable of producing “livingpolymers” which will continue growing as long as substrate monomermolecules are still available. The reactions are, however, relativelyslow compared with the chain reactions of the free radical process.Furthermore, although the quantum yield in terms of reacted molecules istheoretically near infinite, the slow reactions limit spatial resolutionby reason of diffusion of active species out of the imaged area.

A limitation in terms of photopolymer imaging has always been the amountof time needed to deliver an adequate amount of energy to the area to beimaged. The delivery of a large amount of energy is easy. High intensitysources of radiation, simple reflectors and conveyor belts used incombination enable this aim to be achieved. For imaging, the radiationneeds to be collimated and delivered in a controlled fashion. Tocollimate the output from any lamp involves a substantial loss ofintensity. The subsequent use of optical components and even phototoolsserves to reduce the energy from even a very powerful source to aremarkably low level.

It is within this environment that the usage of lasers for imaging hasdeveloped. Although the radiant flux that such lasers will deliver isrelatively low, the intrinsic collimation and the intensity of photonsdelivered at a given wavelength, make the laser a useful light source.Computer guided beam manipulation in combination with mirrors enablesone to eliminate photo tool usage, and further enhances the number ofphotons available for the photochemistry. Nevertheless, theseimprovements have been only incremental, and laser imaged photopolymerprocesses are still slow.

In the silver halide process, the actual efficiency of thephotochemistry is relatively low compared with chain reaction processes.Each photon produces only a single silver atom, thus the quantumefficiency is only one (or, in practice, less than one). The overallsensitivity of the silver process to light only becomes obvious byvirtue of the development step when many more silver atoms are produced.Wherever a silver atom has been produced by irradiation with light, thesilver available accelerates the development reaction, via anautocatalytic reaction, which in turn produces more silver. Thus thequantum efficiency of “image formation” can be varied from one toinfinity, owing to the propagation that occurs in this second stage.However, the propagation occurs only within a grain boundary, viz. thesilver atom produced in one grain can be completely developed butadjacent grains are not developed. Resolution of image details is,therefore, limited by grain size in the silver halide process.

Bradley et al Journal of Photochemistry and Photobiology A: chemistry100 (1996) 109-118 describes the development of vinyl dioxolane basedmonomers as a more amenable alternative to vinyl ethers conventionallyused as monomers for cationic UV curing. Such a material is(2,2′-diphenyl-4-methylene-1,3-dioxolane).

EP-A-1307783 describes a process in which a protected (also referred toas “blocked” or “latent”) photoinitiator is included in a reactivesubstrate. The protected photoinitiator is deprotected in situ and isavailable for a subsequent photoinitiated reaction. The protectedphotoinitiator is a protected ketone photoinitiator in which the ketonegroup is protected by a methylene 1,3 dioxolane group.

It has been found that the prior art ketone photoinitiators having amethylene-1,3-dioxolane moiety are not completely stable and decomposeat room temperature or in the presence of light or very small quantitiesof even quite weak acid. The manufacture, handling and storage ofphotoinitiators based on such materials in commercial quantities maythus be difficult. A need exists for a more stable material which wouldstill function as a blocked initiator under the correct conditions.

SUMMARY OF THE INVENTION

It has now unexpectedly been found that the stability problems of theprotected photoinitiators of EP-1307783 can be overcome by the use of anunsubstituted 1,3 dioxolane protecting group on the ketonephotoinitiator. Ketone photoinitiators protected with an unsubstituted1,3 dioxolane group have been found to be stable in the presence of weakacids and light, but to rapidly deprotect in the presence of strongacid. Such advantageous properties were not found in respect of otherdioxolane protecting groups tested, in particular where the protectinggroup was 4-chloromethyl dioxolane or 4-methyl dioxolane. For suchprotecting groups, the unblocking by deketalization proceeds too slowlyto be effective.

Thus, in accordance with one aspect of the present invention, there isprovided a method for the photoinitiated transformation of atransformable reactive substrate, for example a substrate whichcomprises polymerisable constituents, and/or cross-linkable constituentsand/or colour-changeable constituents. In the method, a protected ketonephotoinitiator species is included in the substrate, and the methodincludes a step in which the protected ketone photoinitiator isdeprotected in situ to form the corresponding ketone photoinitiatorspecies for use in a subsequent photo reaction or photoinitiatedreaction in the method. As noted above, in the present invention, theketone group of the photoinitiator is protected by an unsubstituted 1,3dioxolane group.

In accordance with another aspect of the invention, there is provided amethod for the photoinitiated transformation of a transformable reactivesubstrate (for example the polymerisation of a polymerisable substrateand/or the cross-linking of a cross-linkable substrate and/or the colourchange of a colour-changeable substrate), said method comprising:

(a) applying to the surface of a support a coating which comprises thereactive substrate, a protected ketone photoinitiator and one or morespecies capable of forming acid in response to an external stimulus,wherein the ketone group of the photoinitiator is protected by anunsubstituted 1,3 dioxolane group, and wherein the ketonephotoinitiator, when deprotected by said acid, is capable of forming areactive species on exposure to electromagnetic radiation of a suitablewavelength or energy;

(b) applying an external stimulus to said coating to form acid where theexternal stimulus is applied, whereby said acid reacts with and causesdeprotection of the protected ketone photoinitiator, and wherein theexternal stimulus is not effective to generate reactive species from thedeprotected ketone photoinitiator; and

(c) exposing the coating to electromagnetic radiation of a suitablewavelength or energy to generate a reactive species from the ketonephotoinitiator which, directly or indirectly, is capable of initiatingtransformation of the transformable reactive substrate in said regions.

In this aspect of the invention, the protected ketone photoinitiatorand/or the species capable of generating acid may be included in acoating composition with the reactive substrate for application to thesurface. Alternatively, the coating may be applied in more than one stepin which first the reactive substrate is applied to the surface,optionally with one or other of the other components, followed by asubsequent step or steps in which the other component or components aresupplied to form the coating on the surface.

In accordance with a further aspect of the invention, there is provideda method for the photoinitiated transformation of a transformablereactive substrate (for example the polymerisation of a polymerisablesubstrate and/or the cross-linking of a cross-linkable substrate and/orthe colour change of a colour-changeable substrate), said methodcomprising:

(a) applying to the surface of a support a coating which comprises thereactive substrate and a protected ketone photoinitiator, wherein theketone group of the photoinitiator is protected by an unsubstituted 1,3dioxolane group, and wherein the ketone photoinitiator, when deprotectedby acid, is capable of forming a reactive species on exposure toelectromagnetic radiation of a suitable wavelength or energy;

(b) applying an acid to said coating to cause deprotection of theprotected ketone photoinitiator; and

(c) exposing the coating to electromagnetic radiation of a suitablewavelength or energy to generate a reactive species from the ketonephotoinitiator which, directly or indirectly, is capable of initiatingtransformation of the transformable reactive substrate in said regions.

In accordance with another aspect of the invention, there is providedthe use, in a photoinitiated reaction, of a protected ketonephotoinitiator, wherein the ketone group of the photoinitiator isprotected by an unsubstituted 1,3 dioxolane group.

In a yet further aspect of the invention, there is provided acomposition which comprises a transformable reactive substrate, aprotected ketone photoinitiator wherein the ketone group of thephotoinitiator is protected by an unsubstituted 1,3 dioxolane group, andoptionally a species capable of generating acid. The composition mayalso include optionally, various organic and/or inorganic pigments andfiller materials, flow and levelling agents, solvents, diluents,drying/curing agents, cure accelerators, inhibitors, pH bufferingagents, plasticisers, chain transfer agents.

The invention further provides a ketone photoinitiator in which theketone moiety is protected by a 1,3 dioxolane group. The protectedketone photoinitiator may have the formula (I):

wherein R₁ is phenyl or substituted phenyl;

R₂ is phenyl, substituted phenyl or substituted alkyl;

and wherein, when each of R₁ and R₂ is phenyl or substituted phenyl, therespective phenyl groups may optionally be linked by a bridging moietyto form a structure having the following framework:

where X is S or >C=O and where each of the A and B rings may besubstituted;

or wherein R₁ and R₂ may be joined to form a conjugated ring systemwhich is optionally substituted.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment in which a protected photoinitiator isdeprotected and the photoinitiator is subsequently involved in aphotoinitiated reaction.

FIG. 2a-2c illustrate embodiments in which the photoinitiator functionsas a sensitiser.

FIGS. 3a and 3b illustrate embodiments in which acid is generated by aphoto acid generator or a thermal acid generator, and the acid thenfunctions to deprotect the protected photoinitiator.

FIG. 4 illustrates an embodiment in which an acid generator andprotected photoinitiator function in an auto-accelerative manner.

FIG. 5 illustrates a method involving use of the protectedphotoinitiator and two further initiator species.

FIGS. 6 to 9 illustrate photopolymerization methods of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As described above, the present invention provides for the use of aprotected ketone photoinitiator, in which the ketone moiety is protectedby an unsubstituted 1,3 dioxolane group, in photoinitiated reactions,such as those already described in EP-A-1307783. An advantage of the useof the protected ketone photoinitiator of the invention compared to theprotected photoinitiator used in the prior art reference, (in which theketone group is protected by a methylene 1,3 dioxolane group) is thatthe protected photoinitiator used in the present invention is stable atroom temperature and in the presence of light or very small quantitiesof weak acid, which facilitates manufacture and commercial use. Theprotecting group is however easily removable under conditions of strongacid.

As reported in EP-A-1307783, the acid-catalysed photopolymerizationusing a cationic species and the 2,2′-diphenyl-4-methylene-1,3-dioxolanespecies does not proceed via a simple cationic polymerisation but ratheroccurs rapidly via a ROMP-like (Ring Opening Metathesis Polymerisation)process in which a ketone is produced that is to act as photoinitiatorin the second photoreaction.

In the present invention, the methylene group is absent and thisreaction is not possible. It was therefore expected that thedeketalisation reaction to unblock the protected photoinitiator wouldproceed slowly. Indeed, slow unblocking was found where the protectinggroup was a 4-chloromethyl dioxolane or 4-methyl dioxolane group.Instead, and unexpectedly, where the protecting group was anunsubstituted dioxolane group, the reaction was found to be fast in thepresence of strong acids but did not occur in the presence of weak acidsor light.

Without wishing to be bound by theory, the following mechanism ispostulated.

Under anhydrous conditions, the dioxolane protected photoinitiatorcannot decompose by formation of a diol and regeneration of the parentinitiator and must decompose through a different mechanism. This newmechanism, shown above, is very different from the mechanism by which2,2′-diphenyl-4-methylene-1,3-dioxolane decomposes under acidicconditions as described by Bradley et al and in EP-A-1307783.

Protonation of the methylene group on the dioxolane ring, which triggersits decomposition, is not possible because there is no methylene groupin the unsubstituted dioxolanc.

In the absence of any cation-quenching materials, the simple dioxolancbehaves as a ketal protected carbonyl compound and the process generatesa polyether as the dioxolane ring is opened, generating the theoreticalparent ketone for the dioxolane ring. The most obvious decompositionmode is for the dioxolane to form epoxyethane after protonation of oneof the oxygens of the dioxolane ring. The dioxolane ring openingreaction proceeds with formation of a second strained ring structure (anepoxide), which reacts further to cause ring opening, producing thepolyether and more of the ketone that is to act as a photoinitiator in asecond photoreaction and giving back the proton, making the reactionself catalytic as shown above.

Evidence to support this theory has been demonstrated by analyticalmethods using 2-isopropylthioxanthone (ITX), a well known initiator ofphotochemical induced acrylate polymerisation, which has been blockedusing the unsubstituted dioxolane to form the protected (or latent)photoinitiator (DITX). The following findings were made.

(1) When DITX is subjected to chemical ionization mass spectrometry(anhydrous vapour phase which normally gives very stable protonatedmolecular ions), the expected molecular ion is almost non-existent butthere is mostly decomposition by loss of an epoxyethane molecule to giveprotonated ITX.

(2) NMR spectra from experiments with DITX, show that epoxyethane formsa polymer (polyoxyethylene) and nothing else, other than ITX indicatingthat the reason for the higher than expected reactivity of DITX to acidlies in the mechanism of epoxide formation shown above.

Unlike the 2,2′-diphenyl-4-methylene-1,3-dioxolane in the prior art, ithas been found that free radical polymerisation does not occur.

In accordance with the present invention, the protected ketonephotoinitiator is included in a transformable reactive substratecomprising constituents which are capable of transformation in a photoreaction or a photoinitiated reaction. For example the reactivesubstrate may be a substrate which comprises polymerisable constituents,and/or a substrate which comprises cross-linkable constituents and/or asubstrate which comprises colour-changeable constituents. An example ofa suitable substrate is a mixture of acrylate resins and/or monomers.

The reactive substrate may be applied as a coating to the surface of asupport.

The constituents of the reactive substrate are caused to be transformedin a method which includes a step in which the protected ketonephotoinitiator is deprotected in situ to form the corresponding ketonephotoinitiator species for use in a subsequent photo reaction orphotoinitiated reaction in the method.

In an embodiment, the subsequent photoinitiated reaction may involveexposing the reactive substrate with the deprotected ketonephotoinitiator therein to photoreaction conditions whereinelectromagnetic radiation of a suitable wavelength/energy causes thetransformable constituents (eg polymerisable and/or cross-linkableconstituents and/or colour-changeable constituents) in the substrate toundergo transformation (eg polymerisation and/or cross-linking and/orcolour change). In this step, the radiation, for example actinicradiation (such as UV radiation), may be applied as a flood irradiation.

The exposure of the reactive substrate with the deprotected ketonephotoinitiator therein to suitable photoreaction conditions typicallycauses the photoinitiator to form a reactive species, such as a freeradical, but which alternatively may be an excited state of themolecule. This reactive species then directly or indirectly causes thetransformable constituents (eg polymerisable and/or cross-linkableconstituents and/or colour changeable constituents) in the substrate toundergo transformation (eg polymerisation and/or cross-linking and/orcolour change).

For example, in some embodiments, the photoreaction conditions maydirectly lead to formation of free radical species (unimolecular bondcleavage), or may indirectly lead to formation of free radical speciesby interaction with a coinitiator species or synergist present in thereactive substrate (see FIG. 1).

Typically, in embodiments where the photoreaction conditions cause theformation or generation of a free radical species from the ketonephotoinitiator, this free radical species will directly initiatetransformation of the transformable constituents in the reactivesubstrate. Thus, for example, the transformation may be a free radicalpromoted polymerisation.

In other embodiments the deprotected ketone photoinitiator may functionas a sensitiser, in which case it is employed in conjunction with asuitable second photoinitiator or synergist species which is included inthe reactive composition. In these embodiments, photoreaction conditionscause the ketone photoinitiator to be promoted to an excited state. Thisreactive species interacts with the second photoinitiator or synergist,for example by transferring its energy to the other species, whichinitiates (directly or indirectly) transformation of the transformableconstituents in the reactive substrate. In such embodiments, where theketone photoinitiator functions as a sensitiser, it may return to itsground state after interaction and so be available for furtherexcitation under suitable photoreaction conditions.

In these embodiments where the ketone photoinitiator functions as asensitiser, the second photoinitiator may be a radical photoinitiator ora cationic photoinitiator (see FIGS. 2 a, 2 b ad 2 c).

As noted above, the protected ketone photoinitiator is deprotected insitu, in what may be regarded as a first stage of the method of theinvention, and the deprotected ketone photoinitiator is then used in asubsequent photoinitiated reaction.

Deprotection may be accomplished by including in the reactive substratea species which is capable of generating acid in response to an externalstimulus, the acid being effective for deprotecting the protectedphotoinitiator. This external stimulus may be exposure toelectromagnetic radiation of a suitable wavelength or energy which iseffective to cause generation of acid from the acid-generating species(see FIG. 3 a, where “PAG” stands for photo acid generator). However, inprinciple other external stimuli may be employed, such as theapplication of thermal energy to cause generation of acid from a specieswhich is thermally decomposable to yield an acid catalyst for thedeprotection of the protected photoinitiator. An example of such aspecies is a blocked p-toluene sulphonic acid (see FIG. 3b where “TAG”stands for thermal acid generator).

For example, the unsubstituted 1,3-dioxolane group on the protectedphotoinitiator may for example be removed in a photoreaction which takesplace under initial photoreaction conditions, which may involve a lowenergy dosage of radiation, for example actinic radiation (FIG. 3a ).The protected ketone photoinitiator may be deprotected throughout theentirety of the reactive substrate or throughout only portions of thereactive substrate. To achieve deprotection throughout only portions ofthe substrate, this radiation may for example be applied imagewise by alaser or through a suitable phototool (FIG. 3 b, λ₁ applied imagewise.

Where the substrate is applied as a coating on a support, the protectedketone photoinitiator may be deprotected throughout the entire of thesupport or throughout only portions of the support.

Alternatively, deprotection may be achieved by applying an acid, capableof causing deprotection to occur directly, or an acid generating specieswhich is capable of generating acid in response to an external stimulus,onto a surface of the reactive substrate in a separate step, such as byspraying or ink jet printing.

The acid applied directly to the substrate or generated by the acidgenerating species, which is included in or subsequently applied to thesubstrate, is suitable to effect deprotection of the protected ketonephotoinitiator. In this connection, the acid-generating species shouldbe one which is capable of generating an acid, or the acid itself shouldbe, of a suitable strength to deprotect the deprotected ketonephotoinitiator. If necessary, appropriate tests can be carried out tomatch an acid-generating species to the specific ketone photoinitiatorselected.

In some embodiments, the acid-generating species may also play a role asco-initiator in the subsequent photoinitiated reaction involving thedeprotected ketone photoinitiator, where, in the subsequent stage, theexposure to electromagnetic radiation of a suitable wavelength causesthe formation of a reactive species which interacts with thisco-initiator to generate free radicals, capable of initiatingtransformation under suitable conditions of the transformableconstituents in the reactive substrate, and/or of further acid capableof initiating transformation, under cationic conditions, of thetransformable constituents in the reactive substrate and/or causingfurther deprotection of the protected ketone in an auto-accelerativefashion (see FIG. 4). This embodiment is discussed further below.

As previously mentioned, deprotection of the ketone photoinitiator maybe accomplished in an imagewise fashion, for example, using a photomaskor laser, that is to say the ketone photoinitiator is deprotected inselected regions of the surface of the support. This may be effected byexposure of the selected regions of the reactive substrate toelectromagnetic radiation of a suitable wavelength to effectdeprotection, as previously discussed. The result is that the ketonephotoinitiator is only deprotected in the selected regions. This stepmay be conducted at relatively low energy. Alternatively, selectivedeprotection may be achieved by applying an acid and/or acid generatingspecies to the surface of the substrate in a separate imagewise step,such as by ink jet printing, so that the acid, and/or the acid generatedby the acid generating species in response to exposure to a suitableexternal stimulus, causes deprotection to occur in areas correspondingto the areas to which the acid and/or acid generating species have beenapplied. In the subsequent stage, exposure to electromagnetic radiationof a suitable wavelength which is different to the wavelength of theelectromagnetic radiation used in the initial step where deprotectionwas carried out photo chemically, and is matched to the ketonephotoinitiator will lead to transformation of the transformableconstituents in the reactive substrate only in the selected regions.Exposure to electromagnetic radiation in this step may be high energy toaccomplish rapid curing.

In an alternative approach, the protected ketone photoinitiator may bedeprotected in the first stage over the entire surface of the support.Subsequent exposure to electromagnetic radiation of a suitablewavelength matched to the ketone photoinitiator is then carried outimagewise in selected regions of the surface of the support, for exampleusing a photomask or laser. This alternative approach allows, forexample, a preliminary step to be carried out in which the reactivesubstrate is prepared under conditions (for example radiation such asmay be used in drying) which might otherwise be effective to activatethe ketone photoinitiator, if deprotected. However, because the ketonephotoinitiator is protected at this stage, it is not available to beactivated and so any substantial undesired transformation of thetransformable constituents in the reactive substrate is avoided.

The present invention also relates to a ketone photoinitiator in whichthe ketone moiety is protected by a 1,3 dioxolane group. The protectedketone photoinitiator may have the formula (I):

wherein R₁ is phenyl or substituted phenyl;

R₂ is phenyl, substituted phenyl or substituted alkyl;

and wherein, when each of R₁ and R₂ is phenyl or substituted phenyl, therespective phenyl groups may optionally be linked by a bridging moietyto form a structure having the following framework:

where X is S or >C=O and where each of the A and B rings may besubstituted;

or wherein R₁ and R₂ may be joined to form a conjugated ring systemwhich is optionally substituted.

In one embodiment, the protected ketone photoinitiator has the formula:

where d is 1 to 5 and where each R₃ is independently hydrogen or asubstituent selected from alkyl (for example C1-4 alkyl), aryl (forexample phenyl or substituted phenyl), alkylthio (eg C1-4 alkyl thio),aryl thio (for example where the aryl is phenyl or substituted phenyl,eg substituted by alkyl such as C1-4 alkyl) or heterocycle (for examplemorpholino); and

R₂ is phenyl or substituted phenyl or substituted alkyl.

Where R₂ is substituted phenyl the substituents on the phenyl may beselected from alkyl, or substituted alkyl, phenyl or substituted phenyl.

Where R₂ is substituted alkyl, the substituents may be selected from oneor more of hydroxy, hydroxyalkyl, alkoxy, aryl, alkylaryl, amino,heterocycle, such as morpholino. For example, R₂ may be

in which R_(4,) R₅ and R₆ are each independently selected from hydrogen,hydroxy, hydroxyalkyl, alkoxy, alkyl, aryl, amino or heteroaryl.

Alternatively, R₁ and R₂ may be joined to form a conjugated ring systemin which the ketone of the base photoinitiator is in conjugation with atleast one aromatic ring. For example, R₁ and R₂ may be joined togetherto form a conjugated heterocyclic ring system such as a fluorone ringsystem having the structure:

which may optionally be substituted.

As previously noted, the method of the invention is applicable toreactive substrates wherein the substrate comprises colour-changeableconstituents, which includes chromophores per se. However, in currentapplications it is envisaged that the colour change will take placeduring a crosslinking and/or polymerisation process to enable areas atwhich reaction has taken place to be identified. Leuco crystal violet isan important chromophore in this respect (latent colour formers aregenerally termed “leuco dyes”). Other leuco dyes to which the method ofthe invention may be applied include leucoxanthene and leucofluorans.

With reference now to FIG. 5, and as discussed generally above, theprotected, or latent, photoinitiator is most commonly to be a protectedphotoinitiator (initiator A) whose protection is removed in a reactionwhich takes place under preliminary conditions. Such preliminaryconditions may include the use of another photoinitiator (initiator B),which, when exposed to electromagnetic radiation of an appropriatewavelength/energy, is able to interact with the latent photoinitiator tocause the latent photoinitiator to be converted into initiator A. Thepresent invention will, unless otherwise indicated, be described withreference to such a mode of activating the latent photoinitiator. Thereis then overall use of two photoreactions which will need to includeapplication of electromagnetic radiation at two distinct wavelengths. Inparticular a low power source of radiation may be employed in a first orpreliminary photoreaction, which may be utilised in achieving animagewise exposure of the substrate, and a higher dosage of radiation isapplied subsequently as a flood in connection with a secondphotoreaction in which the reactive substrate is transformed.Preferably, a laser is used in applying the low energy source of actinicradiation imagewise. In contrast with the silver halide processdescribed above, in which the quantum efficiency for the formation ofsilver atoms during the initial exposure stage is one or less; in thecurrent invention, the quantum efficiency for the formation of initiatorA, under preliminary photoreaction reactions, can be much higher thanone. viz. one photon causes many protected initiator molecules to beunblocked, thereby creating many molecules of initiator A. Subsequentexposure of the substrate to actinic radiation in the second step causesinitiator A to polymerise and/or crosslink the substrate and/or cause acolour change to occur in the substrate. Furthermore, by use of acarefully selected combination of initiator A and the wavelength ofradiation used in the second step, initiator A can be made to interactwith a third initiator (initiator C), also applied with the substrateand which does not itself absorb at the wavelength of the radiation usedin the first or second step, via a sensitisation mechanism, whereininitiator C can also cause polymerisation and/or crosslinking and/orcolour change to occur in the substrate as well as further unblocking ofprotected initiator A.

Such method embodying this invention thus also uses an auto-accelerativeamplification step, in which a second photochemical reaction is carriedout subsequently to a preliminary, preferably photochemical reaction.

The unsubstituted 1,3 dioxolane structure can be produced from a ketonephotoinitiator starting material by the skilled person usingconventional chemistry. Reference in this respect may be made to J.Chem. Soc., Perkin Trans. 1, 2001, 1807-1815 and Tetrahedron Letters 42(2001) 1789-1791.

The dioxolane ring opening reaction is applicable to formation of awhole range of ketonic functional initiator and co-initiator speciesranging from such simple well known materials as benzil dimethyl ketalthrough to exotic materials such as di-iodo butoxy fluorone (a viableactive photoinitiator used for stereolithography). When made in the formof the required unsubstituted 1,3 dioxolanes, the resulting compoundsact as “latent photoinitiators”, capable of being activated by thermalacid generators or by low doses of light on formation of catalyticquantities of acid from cationic initiators. Examples of such ketonephotoinitiators are:

Other ketone photoinitiators which may be used as the basis forpreparing a protected photoinitiator are diphenyl ketone,diethoxyacetophenone, 1-hydroxy-cyclohexyl-phenyl ketone,2-hydroxy-2-methyl-l-phenyl-propan-1 -one, various benzoin ethers, aswell as diethoxyacetophenone and 1-hydroxycyclohexyl phenyl ketone, and2-hydroxy-2-methyl-1-phenylpropan-1-one and ethers thereof.

By way of example, the ketone photoinitiator which is to be protected inaccordance with the present invention may ⁻be selected from thefollowing publications, which may overlap in their disclosure. Anyreference to a species in these lists which is not a ketonephotoinitiator is unintended, and should he disregarded:

Photoinitiators disclosed in U.S. Pat. No. 7,585,611B

benzoin and benzoin alkyl ethers such as benzoin, benzoin methyl ether,benzoin ethyl ether, and benzoin isopropyl ether;

acetophenones such as acetophenone, 2,2-dimethoxy-2-phenyl acetophenone,2,2-diethoxy-2-phenyi aectophenone, and 1,1-dichioroacetophenone;aminoacetophenones such as2-methyl-1-[4-methylthio)phenyl]-2-morpholinoaminopropanone-1,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)butan-1-one,and N,N-dimethyl aminoacetophenone:

anthraquinones such as 2-methylanthraquinone, 2-ethylanthraquinone,2-t-butyl-anthraquinone, and 1-chloroanthraquinone;

thioxanthones such as 2,4-dimethylthioxanthone, 2,4-diethyhhioxanthone,2-chlorothioxanthone and 2,4-diisopropylthioxanthone;

ketals such as acetophenone dimethyl ketal and benzyl dimethyl ketal;

benzophenones or xanthones such as benzophenone and4,4′-bisdiethylaminobenzophenone;

2,4,6-tritnethylbenzoyl diphenyl phosphine oxide.

Photoinitiators disclosed in EP-1487888A Benzoins;

benzoin ethers, such as benzoin, benzoin methyl ether, benzoin ethylether and benzoin isopropyl ether, benzoin phenyl ether and benzoinacetate;

acetophenones, such as acetophenone, 2,2-dirnethylacetophenone and 1,1-dichloroacetophenone;

benzil, benzil ketals, such as benzil dimethyl ketal and benzil diethylketal;

2-methyl-1-(4 methylthiophenyl)-2-morpholino-1-propanones which arecommercially available under the name Irgacure®;

anthraquinones, such as 2-methylanthraquinone, 2-ethylanthraquinone,2-tert-butylanthraquinone, 1-chloroanthraquinone, and2-amylanthraquinone, triphenylphosphine, benzoylphosphine oxide (LuzirinTPO, BASF);

benzophenones, such as benzophenone and4,4′-bis(N,N-dirnethylamino)benzophenone, thioxanthones and xanthones,1-phenyl-1,2-propandione 2-O-benzoyloxime, 1-aminophenyl ketones;

1-hydroxyphenyl ketones, such as 1-hydroxycyclohexyl phenyl ketone,phenyl 1-hydroxyisopropyl ketone and 4-isopropyl phenyl1-hydroxyisopropyl ketone, and 2-benzyl-2,2-dimtethylamnino-1-(4-N-morpholinophenyl)-1-butanone,

Photoinitiators disclosed in U.S. Pat. No. 7,425,585B

A photoinitiator of the formula:

R₁ is linear or branched C₁-C₁₂ alkyl,

R₂ is linear or branched C₁-C₄ alkyl,

R₃ and R₄ independently of one another are linear or branched C₁-C₃alkyl

C₁-C₁₂ alkyl is linear or branched and is for example C₁-C₁₀, C₁-C₈,C₁-C₆, C₁-C₄, C₆-C₁₀, C₈-C₁₀, C₆-C₈, C₄-C₈ or C₄-C₁₀ alkyl. Examples aremethyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, isobutyl,tert-butyl, pentyl, hexyl, heptyl, 2,4,4-trimethylpentyl, 2-ethylhexyl,octyl, nonyl, decyl and dodecyl.

R₁ is for example linear or branched C₁-C₄ alkyl, in particular methyl,ethyl, isopropyl, n-propyl, isobutyl and, n-butyl, R₂ is for examplemethyl, ethyl or propyl, particular ethyl and R₃ and R₄ in particularindependently of one another are linear or branched C₁-C₄ alkyl, inparticular methyl.

Examples are

Further examples of photoinitiators disclosed in U.S. Pat. No.7,425,585B are

1. Thioxanthones

Thioxanthone, 2-isopropylthioxanthone, 2-chlorothioxanlhone,2-dodecytlhioxanthone, 2,4-diethylthioxanthone,2,4-dimethylthioxanthone, 1-methoxycarbonylthioxanthone,2-ethoxycarbonylthioxanthone, 3-(2-inethoxyclhoxycarbonyl)-thioxanthone4-butoxycarbonylthioxanthone, 3-butoxycarbonyl-7-methylthioxanthone,1-cyano-3-chlorothioxanthone, 1-ethoxycarbyl-3-chlorothioxanthone,1-ethoxycarbonyl-3-ethoxythioxanthone,1-ethoxycarbonyl-3-aminothioxanthone,1-ethoxycarbonyl-3-phenylsulfurylthioxanthone,3,4-di-[2-(2-methoxyetboxy)-ethoxycarbonyl]-thioxanthone,1-ethoxyeurbonyl-3-(1-methyl-1-morpholinoethyl)-thioxanthone ,2-methyl-6-dimethoxymethyl-thioxanthone,2-methyl-6-(1,1-dimethoxybenzyl)-thioxanthone,2-morpholinomethylthioxanthone, 2-methyl-6-morpholinomethylthioxanthone,N-allylthioxanthone-3,4-dicarboximide,N-octylthioxanthone-3,4-dicarboximide,N-(1,1,3,3-tetramethylbutyp-thioxanthone-3,4-dicarboximide,1-phenoxythioxanthone, 6-ethoxycarbonyl-2-methoxythioxanthone,6-ethoxycarbonyl-2-methylthioxanthone, thioxanthone-2-carboxylic acidpolyethyleneglycol ester,2-hydroxy-3-(3,4-dimethyl-9-oxo-9H-thioxanthon-2-yloxy)-N,N,N-trimethyl-1-propanaminium chloride;

Benzophenones

benzophenone, 4-phenyl benzophenone, 4-methoxy benzophenone,4,4′-dimethoxy benzophenone, 4,4′-dimethyl benzophenone,4,4′-dichlorobenzophenone 4,4′-bis(dimethylamnino)-benzophenone,4,4′-bis(diethylamino)benzophenone, 4-methyl benzophenone,2,4,6-trimethylbenzophenone, 4-(4-methethylthiophonyl)-benzophenone,3,3′-dimethyl-4-methoxy benzophenone, methyl-2-benzoylbenzoate,4-(2-hydroxyethylthio)-benzophenone, 4-(4-tolylthio)-benzophenone,4-benzoyl-N,N,N-trimethylbenzenemethanaminium chloride,2-hydroxy-3-(4-benzoylphenoxy)-N,N,N-trimethyl-1-propanaminium chloridemonohydrate, 4-(13-acryloyl-1,4,7,10,13-pentaoxatridecyl)-benzophenone,4-benzoyl-N,N-dimethyl-N-dimethyl-N-[2-(1-oxo-2-propenyl)oxy]ethyl-benzenemethanaminiumchloride;

3. Coumarins

Coumarin 1, Coumarin 2, Coumarin 6, Coumarin 7, Coumarin 30, Coumarin102, Coumarin 106, Coumarin 138, Coumarin 152, Coumarin 153, Coumarin307, Coumarin 314, Coumarin 314T, Coumarin 334, Coumarin 337, Coumarin500, 3-benzoyl coumarin, 3-benzoyl-7-methoxycoumarin,3-benzoyl-5,7-dimethoxycoumarin, 3-benzoyl-5,7-dipropoxycoumarin,3-benzoyl-6,8-dichlorocoumarin, 3-benzoyl-6-chloro-coumarin3,3′-carbonyl-bis[5,7-di(propoxy)coumarin],3,3′-carbanyl-bis(7-methoxycoumarin),3,3′-carbonyl-bis(7-diethylamino-coumarin), 3-isobutyroylcoumarin,3-benzoyl-5,7-dimethoxy-coumarin, 3-benzoyl-5,7-diethoxy-coumarin,3-benzoyl-5,7-dibutoxycoumarin,3-benzoyl-5,7-di(methoxyethoxy)-coumarin,3-benzoyl-5,7-di(allyloxy)coumarin, 3-benzoyl-7-dimethylaminocoumarin,3-7enzoyl-7-diethylaminocoumarim 3-isobutyroyl-7-dimethylaminocoumarin,5,7-dimethoxy-3-(1-naphthoyl)-coumarin,5,7-diethioxy-3-(1-naphthoyl)-coumarin, 3-benzoylbenzo[f]coumarin,7-diethylamino-3-thienoylcoumarin,3-(4-cyanobenzoyl)-5,7-dimethoxycoumarin,3-(4-cyanobenzoyl)-5,7-dipropoxycoumarin,7-dimethylamino-3-phenylcoumarin, 7-diethylamino-3-phenylcoumarin, thecoumarin derivatives disclosed in JP 09-179299-A and JP 09-325209-A forexample7-[{4-chloro-6-(diethylamino)-S-triazine-2-yl}amino]-3-phenylcoumarin;

4. 3-(aroylmethylene)-thiazolines

3-methyl-2-berizoylmethylene-β-naptithothiazoline,3-methyl-2-benzoylinethylene-benzothiazoline,3-ethyl-2-propionylmethylene-β-naphthothiazoline;

5. Rhodanines

4-dimethylaminobenzalrhodanine, 4-diethylaiminobenzalrhodanine,3-ethyl-5-(3-octyl-2-benzothiazolinylidene)-rhodanine, the rhodaninederivatives, formulae [1], [2], [7], disclosed in JP 08-305019A;

6. Other Compounds

acetophenone, 3-methoxyacetophenone, 4-phenylacetophenone, benzil,4,4′-bis(dimethylamino)benzil, 2-acetylnaphthalene, 2-naphthaldehyde,dansyl acid derivatives, 9,10-anthraquinone, anthracene, pyrene,aminopyrene, perylene, phenanthrene, phenanthrenequinone, 9-fluorenone,dibenzosuberorie, curcumin, xanthone, thiomichler's ketone,α-(4-dimethylaminobenzylidene) ketones, e.g.2,5-bis(4-diethylaminobenzylidene)cyclopentanone,2-(4-dimethylaminobenzylidene)-indan-1-one,3-(4-dimethylamino-phenyl)-1-indan-5-yl-propenone,3-phenylthiophthalimide, N-methyl -3,5 -di(ethylthio)--phthalimide,N-methyl-3,5 (ethylthio)phthalimide, phenothiazine, methylphenothiazine,amines, e.g. N-phenylglycine, ethyl 4-dimethylaminobenzoate, butoxyethyl4-dimethyiaminobenzoate, 4-dimethylaminoacetophenone, triethanolamine,methyldiethanolamine, dimethylaminoethanol, 2-(dimethylamino)ethylbenzoate, p-dimethylaminobenzoate,

An example of a photopolymerisation method embodying this inventiontypically involves an initial irradiation of a film containing acationic (acid producing) photoinitiator, acrylate and unsubstituted 1,3dioxolane-based latent photoinitiator. This irradiation is fast andefficient, as the polymer formed in this step may be minimal and notcause any vitrification, which is a prime limit on reaction rates. Theacid produced by the cationic initiator causes the dioxolane group to beremoved and the underlying photoinitiator to be generated. Subsequent tothe low energy image-wise exposure, the system can be flood irradiatedwith light of wavelength outside the absorption range of the cationicinitiator so as to avoid further photolysis and acid formation, butwithin the absorption range of the initiator formed from the cleavage ofthe dioxolane ring, such that the initiator so formed can causepolymerisation and/or crosslinking and/or colour change to occur in thesubstrate where it has been formed. This irradiation is not image-wiseand therefore can involve far higher dosages of light in a relativelyshort time. An application for this example is that of laser directimaging of photoimageable coatings. The technology eliminates thebottleneck of having to deliver all of the polymerisation energy via thelaser in an image-wise fashion. An example of this technology issummarised in the flow scheme of FIG. 6 of the accompanying drawings.

Preferably in general, cationic acid-producing species such assulphonium and iodonium salts and salt-form organometallic compounds areutilised in achieving conversion of the protected photoinitiatoralthough a-sulphonyloxyketones can also be used as cationicacid-producing species. These compounds are exemplified by thefollowing:

bis[4-(diphenylsulphonio)-phenyl]sulphide bis-hexafluorophosphate orbis-hexafluoroantimonate which may optionally be in combination with amono- or poly-[4-(phenylthiodiphenyl)] sulphonium hexafluorophosphate orhexafluoroantimonate;

bis[4-(di(4-(2-hydroxyethyl)phenyl) sulphonio-phenyl] sulphidebis-hexafluorophosphate;

bis[4-(di(4-(2-hydroxyethyl)phenyl) sulphonio)-phenyl] sulphidebis-hexafluoroantimonate;

(η⁶-2,4-(cyclopentadienyl)[(1,2,3,4,5,6-η)-(methylethyl)benzene]-iron(II) hexafluorophosphate;

4-isopropyl-4-mcthyl diphcnyliodonium hcxafluorophosphate;diphenyliodonium hexafluorophosphate;

4-isopropyl-4-methyl diphenyliodoniumtetrakis-(penta-fluorophenyl)borate;

diphenyliodonium tetrakis-(penta-fluorophenyl)borate and;

2′-hydroxy-2-phenyl-3-toluenesulphonyloxypropiophenone.

A wide range of formulations is possible. Preferably, for theacid-producing photoinitiator, the amount employed is up to 5% byweight, for example in the range of from 0.25 to 5% by weight of thematerial to be acted on. For the protected photoinitiator, the practicalworking range is up to 10%, for example from 0.25 to 10% by weight ofthe substrate to be acted on. In an embodiment, the amount of protectedphotoinitiator may be in the range of 3-10% by weight of the substrate.

While it is unlikely that this process will be able to completely matchthe overall silver halide process in terms of photosensitivity,nonetheless it permits products to be obtained which dramaticallyincrease the productivity of imaging processes dependent on curing byelectromagnetic radiation. Practical experience has shown that it isparticularly convenient to work in the UV range. With the unsubstituted1,3 dioxolane latent photoinitiators and the aforementionedacid-producing photoinitiators, one can use UV irradiation of arelatively short wavelength in the first photochemical reaction and UVirradiation of a longer wavelength in the second photochemical reaction,or vice versa.

A second procedure embodying this procedure is in the field ofphotoimageable inks for sequential build-up (SBU) where cationic systemsare to be preferred due to their advantageous physical properties. Thisprocedure demonstrates the versatility of the method of the invention.One of the materials which can be made in protected form where theketone is protected by an unsubstituted dioxolane ring isisopropylthioxanthone (ITX). Thioxanthones are particularly suitable assensitizers for iodonium salts. As the iodonium salts themselves aregenerators of acid catalysts for cationic polymerisation, thepossibility of their being used in a self-sensitizing system is apparentand provides a means of increasing quantum yields. Initial irradiationof the iodonium salt directly in an image-wise fashion results inproduction of a small amount of acid polymerisation catalyst. Theresulting film is sensitive to an auto-accelerative reaction when floodirradiated with near visible radiation. This enhanced sensitivity opensthe way for using laser direct imaging with photoimageable SBUdielectric. An example of this technology is summarised in the flowscheme of FIG. 7 of the accompanying drawings.

As the number of boards produced by the printed circuit board industryhas increased, so have concerns over the environmental impact of boardmanufacturing processes, especially with regards to the emission ofvapours which are difficult to contain or collect and reprocess. Aprocess responsible for generating organic vapour emissions is theapplication, by various coating methods, of Liquid Photoimageable Soldermasks (LPISM). In this process, the PCB is completely coated with asolvent-containing liquid formulation. After coating, the boards aredried in an oven to evaporate the solvent and produce a tack-free,photosensitive coating. Image-wise exposure of the coating andsubsequent developing, either by aqueous carbonate or organic solventsallows formation of openings in the mask for purposes of component orconnector placement. This technology is summarised in the flow scheme ofFIG. 8 of the accompanying drawings. In the face of increasinglystringent regulatory requirements, control technology to reduce oreliminate vapour emissions, particularly during the drying stage, willbecome necessary. A few manufacturers have ventured to introducewaterbased LPISMs into the market place but these appear to betechnically inferior to traditional solvent based products.

Use of a photoacid generating initiator (cationic initiator) inconjunction with a dioxolane blocked free radical initiator permitsformulation of a 100% solids LPISM. This system has the advantage ofcontaining no solvent and therefore producing no emissions. Such a 100%solids formulation, which can be made tack-free on exposure to acidcatalysts, but whilst remaining developable in a suitable medium, can bemade by following the teaching of the current invention. The LPISM thusproduced can be UV dried, being heated to complete the solidificationprocess and to ensure complete deblocking. Subsequently it can bere-exposed to image the formulation and still be developed and finallycured in a fashion familiar to the industry. The use of dioxolaneblocked latent photoinitiators preserves the radical initiator duringthe imaging step whilst the coating is UV dried. The process sequence isshown by the flow diagram in FIG. 9.

A variety of approaches can be used to effect the UV drying. Vinylethers, cycloaliphatic epoxides and oxetane compounds can be used aspolymerisable solvents. Alternatively, use of such materials as crosslinkers for functionalised resins is viable. Use of reactive resins, andbuilding resin molecular weight by cationic reaction can also be used.Finally, it is possible to use a vinyl ether functional resin and reactit under cationic conditions with a hydroxy functional solvent or viceversa. Such technology may be represented by the following:

Similar applications are found in the manufacture of flexographicprinting plates and other imaged printing systems.

A further example of the use of latent photoinitiators is in the fieldof visible light active photoinitiators. The use of these initiatorsrequires the working areas to be configured as “red light” zones, whichprovide an exceedingly difficult environment to work in. By using theuse of this invention to produce blocked, visible light photoinitiators,formulations can be produced, which become sensitive to visible lightonly after activation. Substantial production and handling benefitsensue from this.

Yet another application is in the curing of thick clear films inconjunction with high intensity excimer lamp technology. A lowconcentration of initiator, whilst necessary for minimising opticaldensity in thick coatings, is not normally adequate to effectpolymerisation. Thus the choice of photoinitiator used for the curing ofthick films is limited to those which undergo photobleaching e.g.acylphosphine oxides. With careful formulation, use of a lowconcentration of cationic initiator in conjunction with a dioxolaneblocked free radical initiator, results in the ongoing, in-situformation of free radical initiator during irradiation by UV light.Thus, because the light absorption of dioxolane protected initiators isat shorter wavelength than their parent initiator, optical density canbe controlled with only a small amount of free radical initiator beingpresent at any time. High intensity excimer lamps are particularlysuitable for this application as the near monochromatic output of theexcimer lamps is capable of photolysing low concentrations of shortwavelength sensitive cationic initiators in very thick clear films.

The latent initiators described here are not confined to being unblockedby photochemical processes yielding acid moieties. Any blocked acid ofsufficient strength, for example a blocked toluenesulphonic acid,especially p-toluenesulphonic acid, will, on unblocking, catalyse thebreakdown of the dioxolane ring on the latent photoinitiators. Examplesof such compounds that can be used are the blocked superacids producedby King Industries. These can be unblocked thermally and can be usefulfor a number of the examples given here, especially the 100% solidsLPISM and give improved handling for viable initiated processes.

In summary, the chemistry outlined here provides for the synthesis of ablocked photoinitiator by a prescribed synthetic method, for example byreaction of a carbonyl group to form a dioxolane ring, as latentphotoinitiator.

The technology is useful in the following applications:

1. Primary and secondary imaging (PCB industry).

2. UV initiated sensitisation of visible initiators.

3. Making visible active formulations easy to handle.

4. Controlling the cure of thick films.

5. Improving shelf life of UV curing formulations.

EXAMPLES Example 1

Preparation of 1,3 dioxolane protected 2-isopropylthioxanthone (DITX)

To a 5000 ml, 3-necked, round-bottomed flask equipped with overheadmechanical stirrer, reflux condenser connected to a sodium hydroxidescrubber and Nitrogen inlet was dissolved a solution of 2-ITX (700 g,2.752 M) in Thionyl chloride (3262 g, 2000 ml, 27.42 M, 10 Mol eq's).Under a Nitrogen stream this wine-red solution was heated to reflux(internal temp=80° C.) with stirring for 4.5 hours. The reaction mixturewas cooled to ambient overnight under a positive pressure of Nitrogen.The wine red reaction mixture was transferred to a rotorvapor and excessThionyl chloride was removed under reduced pressure (water bathtemperature was maintained at 40° C.). The mobile oil was taken up inToluene (1050 ml) and then added drop wise, under a Nitrogen atmosphere,over 1.5 hour, to a previously prepared and vigorously stirred solutionof Sodium methoxide (595 g, 11.014 M, 4.0 Mol eq's) in Methanol (2100ml) at such a rate as to keep the vessel temperature between 0-10° C.Once addition was complete, the cooling bath was removed and the vesselcontents allowed to warm to ambient over 1 hour.

Tap water was added to the cooling bath and stirring continued for afurther hour. The reaction mixture was then transferred to a rotorvaporand Methanol removed under reduced pressure. The resulting residue wasquenched into a vigorously stirred mixture of Toluene (1.0 L) and Water(3.5 L). Stirring was continued for 20 minutes at ambient and thereaction mixture was then transferred to a separating funnel and allowedto settle over 15 minutes. The Toluene layer was back washed with Water(1.75 L) and allowed to separate. The upper Toluene layer was removedfrom the separator, dried over sodium sulphate and stored in the freezerovernight. The Toluene solution was then filtered through Celite and theresulting orange/brown filtrate was transferred to a rotorvapor and

Toluene removed under reduced pressure to give crude9,9-Dimethoxy-2isopropylthioxanthone as a pale brown oil.

To the crude 9,9-Dimethoxy-2-1TX oil (2848 g, 9.48 M) was added THF (2.4L), camphor sulphonic acid (39.0 g, 0.1678 M, 1.7 Mol %) and ethyleneglycol (1536 g, 1380ml, 24.74 M, 2.6 Mol eq) and stirring was commencedat ambient. Stirring was continued over the weekend where a completioncheck revealed that all the 9,9-dimethoxy-2-ITX had been consumed. Thereaction mixture was then transferred to a rotorvapor and organicsremoved under reduced pressure (bath at 40° C.). The cooled distillationresidue was dissolved in DCM (4.0 L) and back washed with 5% Sodiumbicarbonate solution (2×4.0 L) and Water (2×5.0 L). The organic layerwas then dried over sodium sulphate, filtered through Celite, cakewashed with DCM (1×1.0 L) and filtrate stripped to a brown oil on therotorvapor (water bath=40° C. @ 10 mbar).).

The crude oil was purified during work up to remove unreacted startingmaterial, dissolved in Toluene (10 L) and then filtered through basicalumina (3 Kg) (pre-washed with Toluene (10 L)). A total of threefiltrations were carried out where the basic alumina was regeneratedbetween filtrations by washing with DCM (2 L), Methanol (2×2 L) andfinally Toluene (1×5 L). The resulting filtrate was concentrated on therotorvapor to approx half volume, polish filtered through GFIF filterpaper (to remove alumina fines) and then returned to the rotorvapor andfurther concentrated to a pale amber oil (water bath=55° C. @ 15 mbar).Yield of crude DITX oil=2250 g. GC purity indicated 65.5% product (DITX)and 28% reduced alcohol. The crude oil was then warmed to 40° C. inpre-treated Ethanol*(2.5 L) and the resulting solution was allowed tocool, with stirring, to ambient temperature where crystallisation ofDITX was observed. The pale yellow solid was washed with cold ethanol(2×1.0 L) and pulled dry on the filter. The damp weight of pure DITX was1225 kg. The off-white material was air dried in the fume hood over 48hours to give pure DITX in an overall yield of 1078 g, 37.5% from 2-ITX.

Analysis of DITX

Appearance: Off-white solid

Purity (G.C): 99.2%

T.l.c. (Petrol 9:1 Ethyl acetate): Product spot @ Rf=0.40+Two Faintimpurity spots @ Rf=0.07+0.01

Solution (5% DCM): Clear colourless solution

Mp. 70-72° C. (Sharp)

IH nmr: (CDC13): Conforms to structure

CHN: Found: C: 72.45%, H: 6.09%. Theory: C: 72.45%, H: 6.08%.

Example 2

The use of DITX prepared in Example 2 was tested as a protectedphotoinitiator in comparison with a protected ITX in which theprotecting group was a methylene 1,3 dioxolane group was tested and aprotected ITX in which the protecting group was a chloromethylene 1,3dioxolane group.

The test sample of dioxolane was employed at a level of 5% in aformulation which also contained 3% of aryl iodoniumhexafluorophosphate, 80% of an acrylated and carboxylated epoxy novolacresin (65% solids) and 12% of a difunctional acrylic monomer.

The test formulations were coated in duplicate onto standard microscopeslides using a gap-bar type coater with a gap of approximately 200microns. The coater deposited a wet thickness of approximately 100microns.

The UV spectrum of each test sample was recorded before drying, and theabsorbance at 385 nm recorded. All samples were then dried at 80C for 20minutes, and the UV spectrum recorded again. Any increase in absorbanceat 385 nm was noted. This measurement reflects the stability to dryingof the test dioxolane in a formulation.

One slide of each test formulation was then subjected to 6 minutesirradiation using a UV Process Supply 415 nm LED lamp array from adistance of approximately lcm.

Again the UV spectrum of the irradiated samples was recorded and anyincrease in absorbance at 385 nm recorded. This measurement alsoreflects the stability of the dioxolane but is more sensitive as itallows for amplification of any ITX produced. It also demonstratesstability of formulation to 415 nm light.

The second slide for each test formulation which exhibited stability to415 nm irradiation was subjected to 15 seconds exposure to a lowpressure mercury grid lamp radiating principally at 254 nm, positioned20 mm from the coating surface.

The rate of increase of absorbance at 385 nm was measured and the samplesubjected to irradiation at 415 nm for 2 minutes, subsequent to whichthe rate of increase of absorbance at 385 nm was re-measured. Anypositive change in the rate of increase reflects the amplificationeffect.

The results obtained arc set forth in Table 1 below.

Protecting group on ITX methylene 1,3 13, chloromethylene dioxolanedioxolane 1,3 dioxolane Stable to drying No. Yes. Yes. in formulation?Stable to 415 nm? No. (ITX Yes. Yes. formed at drying) Response to N/AYes. Minimal. 254 nm? Amplification by N/A Rapid. Slight. 415 nm? Alltests passed? No. Yes. Yes. (But performance poor)

The methylene 1,3 dioxolane functional blocked initiator is unstable incarboxylic acid containing formulations which may limit its utility forsome applications. The chloromethylene dioxolane demonstrates thatalternative substituted dioxolanes are more stable but have poorperformance. The simple dioxolane is unexpectedly superior in terms ofstability and deketalisation.

1-18. (canceled)
 19. A composition comprising: a transformable reactivesubstrate; a species capable of generating acid; and a protected ketonephotoinitiator, wherein the protected ketone photoinitiator has theformula (I):

wherein Ri is substituted phenyl; and R₂ is phenyl, substituted phenylor substituted alkyl.
 20. The composition according to claim 19,wherein: when R₂ is phenyl or substituted phenyl, the phenyl groups ofR₁ and R₂ are linked by a bridging moiety to form a structure having thefollowing framework:

where X is S or >C=O and where at least one of the A and B rings issubstituted; or wherein R₁ and R₂ are joined to form a conjugated ringsystem which is substituted.
 21. The composition according to claim 19,wherein the protected ketone photoinitiator has the formula:

where d is 1 to 5 and where at least one R₃ is a substituent selectedfrom alkyl, aryl, alkylthio, aryl thio, or heterocycle; and R₂ is phenylor substituted phenyl or substituted alkyl.
 22. The compositionaccording to claim 19, wherein R₁ and R₂ are joined to form a conjugatedring system in which the ketone of the base photoinitiator is inconjugation with at least one aromatic ring.
 23. The compositionaccording to claim 19, wherein R₁ and R₂ are joined together to form aconjugated heterocyclic ring system.
 24. The composition according toclaim 19, wherein the species capable of generating acid generates acidin response to an external stimulus.
 25. The composition according toclaim 19, wherein the transformable reactive substrate is selected from:a substrate comprising polymerisable constituents, and/or a substratecomprising cross-linkable constituents; and/or a substrate comprisingcolour-changeable constituents.
 26. The composition according to claim19, wherein the ketone photoinitiator is any one or more selected fromthe group of substituted acetophenones or anthraquinones.
 27. Thecomposition according to claim 26, wherein the ketone photoinitiator isany one or more selected from 2,2-dimethoxy-2-phenyl acetophenone,2,2-diethoxy-2-phenyl acetophenone, 2,2-dimethylacetophenone and1,1-dichloroacetophenone, 3-methoxyacetophenone, 4-phenylacetophenone.28. The composition according to claim 26, wherein the ketonephotoinitiator is any one or more selected from 2-methylanthraquinone,2-ethylanthraquinone, 2-t-butyl-anthraquinone, 1-chloroanthraquinone2-amylanthraquinone, and 9,10-anthraquinone.
 29. The compositionaccording to claim 19, wherein the ketone photoinitiator is any one ormore selected from the group of thioxanthones or xanthones.
 30. Thecomposition according to claim 29, wherein the ketone photoinitiator isany one or more selected from 2,4-dimethylthioxanthone,2,4-diethylthioxanthone, 2-chlorothioxanthone, 2,4-diisopropylthioxanthone, 2-i sopropylthioxanthone, 2-dodecylthioxanthone,1-methoxycarbonylthioxanthone, 2-ethoxycarbonylthioxanthone,3-(2-methoxyethoxycarbonyl)-thioxanthone, 4-butoxycarbonylthioxanthone,3-butoxycarbonyl-7-methylthioxanthone, 1-cyano-3-chlorothioxanthone,1-ethoxycarbonyl-3-chlorothioxanthone,1-ethoxycarbonyl-3-ethoxythioxanthone,1-ethoxycarbonyl-3-aminothioxanthone,1-ethoxycarbonyl-3-phenylsulfurylthioxanthone,3,4-di-[2-(2-methoxyethoxy)-ethoxycarbonyl]-thioxanthone,1-ethoxycarbonyl-3-(1-methyl-1-morpholinoethyl)-thioxanthone,2-methyl-6-dimethoxymethyl-thioxanthone,2-methyl-6-(1,1-dimethoxybenzyl)-thioxanthone,2-morpholinomethylthioxanthone, 2-methyl-6-morpholinomethylthioxanthone,N-allylthioxanthone-3,4-dicarboximide,N-octylthioxanthone-3,4-dicarboximide,N-(1,1,3,3-tetramethylbutyl)-thioxanthone-3,4-dicarboximide,1-phenoxythioxanthone, 6-ethoxycarbonyl-2-methoxythioxanthone,6-ethoxycarbonyl-2-methylthioxanthone, thioxanthone-2-carboxylic acidpolyethyleneglycol ester,2-hydroxy-3-(3,4-dimethyl-9-oxo-9H-thioxanthon-2-yloxy)-N,N,N-trimethyl-1-propanaminium chloride.
 31. The composition according toclaim 19, wherein the ketone photoinitiator is any one or more selectedfrom the group of 4-phenyl benzophenone, 4-methoxy benzophenone,4,4′-dimethoxy benzophenone, 4,4′-dimethyl benzophenone,4,4′-dichlorobenzophenone 4,4′-bis(dimethylamino)-benzophenone,4,4′-bis(diethylamino)benzophenone, 4-methyl benzophenone,2,4,6-trimethylbenzophenone, 4-(4-methylthiophenyl)-benzophenone,3,3′-dimethyl-4-methoxy benzophenone, methyl-2-benzoylbenzoate,4-(2-hydroxyethylthio)-benzophenone, 4-(4-tolylthio)-benzophenone,4-benzoyl-N,N,N-trimethylbenzenemethanaminium chloride,2-hydroxy-3-(4-benzoylphenoxy)-N,N,N-trimethyl-1-propanaminium chloridemonohydrate, 4-(13-acryloyl-1,4,7,10,13-pentaoxatridecyl)-benzophenone,4-benzoyl-N,N-dimethyl-N-[2-(1-oxo-2-propenyl)oxy]ethyl-benzenemethanaminiumchloride.
 32. The composition according to claim 19, wherein the ketonephotoinitiator is any one or more selected from the group of1-phenyl-1,2-propanedione-2-(O-methoxycarbonyl)oxime, N,N-dimethylaminoacetophenone, 2-methyl-4′-(methylthio)-2-morpholino-propiophenone,9-fluorenone, and di-iodobutoxyfluorone.
 33. The composition accordingto claim 19, wherein the ketone photoinitiator is any one or moreselected from the group of 1-aminophenyl ketones or 1-hydroxyphenylketones.
 34. The composition according to claim 33, wherein the ketonephotoinitiator is any one or more selected from 1-hydroxycyclohexylphenyl ketone, phenyl 1-hydroxyisopropyl ketone, 4-isopropylphenyl(1-hydroxyisopropyl) ketone, and 2-b enzyl-2-dimethylamino-1-(4-N-morpholinophenyl)-1-butanone.
 35. The composition according toclaim 19, wherein the ketone photoinitiator is any one or more selectedfrom the following structure:

wherein R1 is linear or branched C1-C12 alkyl; R2 is linear or branchedC1-C4 alkyl; R3 and R4 independently of one another are linear orbranched C1-C8 alkyl.
 36. The composition according to claim 35, whereinthe ketone photoinitiator comprises one or more of:


37. The composition according to claim 19, wherein the ketonephotoinitiator is any one or more selected from the group of coumarins.38. The composition according to claim 37, wherein the ketonephotoinitiator is any one or more selected from Coumarin 1, Coumarin 2,Coumarin 6, Coumarin 7, Coumarin 30, Coumarin 102, Coumarin 106,Coumarin 138, Coumarin 152, Coumarin 153, Coumarin 307, Coumarin 314,Coumarin 314T, Coumarin 334, Coumarin 337, Coumarin 500, 3-benzoylcoumarin, 3-benzoyl-7-methoxycoumarin, 3-benzoyl-5,7-dimethoxycoumarin,3-benzoyl-5,7-dipropoxycoumarin, 3-benzoyl-6,8-dichlorocoumarin,3-benzoyl-6-chloro-coumarin, 3,3′-carbonyl-bis[5,7-di(propoxy)coumarin],3,3′-carbonyl-bis(7-methoxycoumarin),3,3′-carbonyl-bis(7-diethylamino-coumarin), 3-isobutyroylcoumarin,3-benzoyl-5,7-dimethoxy-coumarin, 3-benzoyl-5,7-diethoxy-coumarin,3-benzoyl-5,7-dibutoxycoumarin,3-benzoyl-5,7-di(methoxyethoxy)-coumarin,3-benzoyl-5,7-di(allyloxy)coumarin, 3-benzoyl-7-dimethylaminocoumarin,3-benzoyl-7-diethylaminocoumarin, 3-isobutyroyl-7-dimethylaminocoumarin,5,7-dimethoxy-3-(1-naphthoyl)-coumarin,5,7-diethoxy-3-(1-naphthoyl)-coumarin, 3-benzoylbenzo[f]coumarin,7-diethylamino-3-thienoylcoumarin,3-(4-cyanobenzoyl)-5,7-dimethoxycoumarin,3-(4-cyanobenzoyl)-5,7-dipropoxycoumarin,7-dimethylamino-3-phenylcoumarin, 7-diethylamino-3-phenylcoumarin,7-[{4-chloro-6-(diethylamino)-S-triazine-2-yl} amino]-3-phenylcoumarin.